Aluminosilicate zeolites are microporous crystalline solids containing cavities and channels of molecular dimensions. These cavities and channels are precisely uniform in size. Since the dimensions of these pores are such that they can accept molecules of certain size while rejecting molecules of larger dimensions, these materials have come to be widely known as "molecular sieves." This property has been utilized in a variety of ways for adsorption, catalysis, ion exchange, purifications and separations.
The primary building block of the zeolite structure is a tetrahedron of four oxygen atoms surrounding a central silicon atom (SiO.sub.4).sup.4-. These tetrahedra are connected through shared oxygen atoms to form a wide range of secondary building units. Different combinations of the secondary building units give rise to numerous distinctive zeolite framework structures with varying pore sizes. In aluminosilicate zeolites, some of the Si.sup.4+ atoms are substituted by Al.sup.3+ atoms, and this results in a single net negative charge on the framework which is compensated by a nonframework cation (e.g., Na.sup.+) that is located in the pores or cavities of the structure. These charge compensating cations are relatively mobile and can, in many cases, be easily exchanged for other cations.
Although naturally occurring zeolites of the "molecular sieve" type were known to mineralogists for many years, the art of synthesizing such materials is relatively recent. Prior art techniques for synthesis, described in patents and open technical literature, have been used in the preparation of a variety of commercially important crystalline aluminosilicate zeolite materials. A representative list of such products and relevant patents is shown in Table 1. Numerous other patents exist on the synthesis of various zeolite products possessing other structures and compositions.
TABLE 1 __________________________________________________________________________ Examples of Commercially Available Natural and Synthetic Zeolites No. of Tetrahedra Natural/ in Largest Pore Zeolite Synthetic Ring Si:Al Cation Size .ANG. U.S. Pat. No. __________________________________________________________________________ 1 Erionite Nat 8 3 Variable 3.6 (Na, K, Ca, Mg) 2 Mordenite Nat 12 5 Variable 3.9 (Na, K, Ca) 3 Chabazite Nat 8 2 Na, K, Mg 3.9 4 A Syn 8 1.0 K, Na, Ca 3-4.5 2,882,243 5 X (Faujasite) Syn 12 1-1.5 Na 7.4 2,882,244 6 Y (Faujasite) Syn 12 1.5-3 Na 7.4 3,130,007 7 Ferrierite Syn 10 5 Na, K, Mg 5.5 3,966,883 8 Mordenite (LP) Syn 12 5 Na 6.7 3,436,174 9 ZSM-5 Syn 10 10-500 Na, H 5.5 3,702,886 __________________________________________________________________________
In the context of the present invention, mordenites and the ZSM-5 series of high Si:Al ratio aluminosilicates are of particular significance. These materials have been widely used as catalysts in organic synthesis. Important examples of their uses include the conversion of methanol to gasoline, the dewaxing of distillates and the interconversion of aromatic compounds. The high Si:Al ratio has been shown to result in hydrophobicity leading to potential applications in the separation of hydrocarbons from polar compounds (e.g., water and alcohols).
Natural mordenite is the most siliceous natural zeolite with a 10:1 SiO.sub.2 /Al.sub.2 O.sub.3 ratio (Formula: Na.sub.2 O.cndot.Al.sub.2 O.sub.3 .cndot.10SiO.sub.2 .cndot.6H.sub.2 O). The framework structure consists of 12- and 8-membered rings formed with 5-membered silica tetrahedron rings. The high degree of thermal stability of mordenite has been attributed to the presence of the 5-membered rings which are energetically favored in terms of stability. A two-dimensional channel system is created by the 8-membered ring system whose dimensions (2.6.times.5.7 .ANG., perpendicular to the b axis) allow the diffusion of small molecules. The 12-membered ring system (perpendicular to the c axis) creates a one-dimensional channel with a dimension of 6.7 .ANG.. In natural mordenite, diffusion blocks produced by stacking faults or by the presence of cations and amorphous material tend to restrict the kinetic diameter of diffusing molecules to only about 3.9 .ANG.. Because this channel system is only in one direction, the number of diffusion blocks needed to restrict the diffusion of large molecules is not large.
On the other hand, synthetic "large port" mordenite exhibits the adsorption characteristics expected for the free diffusion of molecules in the large 6.7 .ANG. diameter channels. The defects and the extraneous matter which are assumed to block the main channels in the natural mineral are apparently not present in the synthetic product.
The ZSM-5 zeolite is normally synthesized by including organic molecules such as tetrapropyl ammonium bromide in the reaction mixture. These organic molecules have been described as functioning as "templates" or "directing agents" around which the zeolite crystallizes. Typical compositions of the reaction mixture and synthesis conditions are given in U.S. Pat. No. 3,702,886. The organic guest molecule is incorporated into the zeolite structure as it is formed. The zeolite may be freed of the organic guest molecule by high temperature treatment without changing its framework topology.
The aluminum content of ZSM-5 type zeolites can be changed by several orders of magnitude with silicon contents approaching substantially pure silica without altering the basic framework configuration.
The framework of ZSM-5 zeolite contains a novel configuration of linked silicon tetrahedra. The channel system is three-dimensional and defined by 10-member rings of tetrahedra consisting of straight and sinusoidal channels with a dimension of 5.5 .ANG..
The products of the present invention, although structurally similar to mordenite and ZSM-5 (as shown by the X-ray diffraction patterns), may be considered as a different class of high silica zeolites because they do not contain the organic "template molecules" and have not been "dealuminized" subsequent to their synthesis.
It is a principal objective of the present invention to provide a method for producing novel crystalline zeolites utilizing an amorphous silica powder that is obtained by neutralization of fluosilicic acid with alumina.
It is a related advantage of the claimed method that the novel high silica zeolites may be prepared without organic template molecules and without being dealuminized subsequent to their synthesis.
Additional objectives and advantages of the present invention will become readily apparent to persons skilled in the art from the following detailed description.